node.js - For 循环在 fs.readfile 完成之前迭代

Question

预期行为：element.urls[j] == url，因此fs.readFile被触发。最终，代码到达fs.writeFile并执行resolve(reply)。

当前行为：代码按预期运行，直到fs.readFile. fs.readFile开始同步运行，这意味着循环在其中的代码块完成执行之前继续迭代。这会导致我的循环过早结束，因此 as fs.writeFileis complete 并尝试执行resolve(reply)，它什么也不做，因为 promise 已经被最后拒绝了reject(reply)，一旦到达数组的末尾就结束循环。

这是我的代码，请见谅console.log()s。正在使用它们进行调试：

    //Loop through monitor and access the "urls" key of each json
    let i = 0;
    for (let element of monitor) {
        //Checks if the url is in the array
        for (j = 0; j < element.urls.length; j++) {
            if (element.urls[j] == url) {
                console.log("yes the url is in the array");
                //The URL is in the array
                let item = monitor[i].product;
                //Removes the URL from the array
                monitor[i].urls.splice(j, 1);
                //Updates config.json with the new data
                console.log("removed the url from the arrauy");
                //Code splits into two branches here for some reason
                fs.readFile('./config.json', function (err, data) {
                    console.log("file is opened time to update");
                    var json = JSON.parse(data);
                    json.monitor = monitor;
                    fs.writeFile("./config.json", JSON.stringify(json), (err) => {
                        console.log("file updated");
                        let reply;
                        if (err) {
                            //There was an error trying to save the data
                            reply = [
                                {
                                    "name": "An error occured trying to remove the url from array",
                                    "value": `Error: ${err}`
                                }
                            ];
                            //Exit
                            reject(reply);
                            return;
                        }
                        //Success
                        reply = [
                            {
                                "name": "Successfully removed url from array",
                                "value": `I will now stop monitoring the url supplied`
                            },
                            {
                                "name": "Item name",
                                "value": item
                            },
                            {
                                "name": "URL supplied",
                                "value": url
                            }
                        ];
                        //Exit
                        resolve(reply);
                        return;
                    });
                });
            } else {
                console.log("monitor length");
                console.log(monitor.length-1);
                console.log("value of i");
                console.log(i);
                console.log("length of elements urls");
                console.log(element.urls.length-1);
                console.log("value of j");
                console.log(j);
                //Check if this is the final iteration
                if (i == monitor.length-1 && j == element.urls.length-1) {
                    console.log("ugh");
                    //This is the last iteration, the url supplied is not monitored by the bot
                    reply = [
                        {
                            "name": "The bot was never monitoring that URL",
                            "value": `The URL supplied isn't one that the bot currently monitors`
                        }
                    ];
                    //Exit
                    reject(reply);
                    return;
                }
            }
        }
        console.log("incrementing");
        //Increment i
        i++;
    };

这是控制台输出，以便可以更好地可视化代码的路径：

monitor length
1
value of i
0
length of elements urls
1
value of j
0
yes the url is in the array
removed the url from the arrauy
incrementing   <----- Shows that the loop is incrementing already
monitor length   <----- Start of the loop again
1
value of i
1
length of elements urls
0
value of j
0
ugh
?????   <----- Comes from outside the function, shows that the function this loop is in has already ended
file is opened time to update   <----- This comes from after fs.writeFile() is done opening the file
file updated   <----- fs.writeFile() is done saving the file to disk

如您所见，第一个循环被分解并fs.readFile()同步运行。

非常感谢任何输入。在过去的几个小时里一直在这样做。

标签： node.jsfor-loop

Answer 1

fs.readFile()是非阻塞的。这意味着您的for循环继续运行，同时它fs.readFile()从您的循环开始所有操作，并且它们都在同一时间进行中。然后，for循环结束，有时稍后您的fs.readFile()操作完成并调用它们的回调，它们可以按任何顺序完成。因此，在您的for循环完成时，fs.readFile()尚未调用任何完成回调，并且可以按任何顺序调用它们。这应该可以解释您在输出中看到的内容。

为了让它们按顺序运行，最简单的方法是切换到使用 Promise 和 readFile 的 Promise 接口。在结构上，这看起来像这样：

// function needs to be async
async function someFunction() {
    for (...) {
        try {
            let data = await fs.promises.readFile(...);
            // do something with data
        } catch(e) {
            // handle error
        }
    }
    // all files have been processed here
}

这将按顺序处理您的文件，暂停for循环，直到每个连续的 readFile() 完成并按顺序运行您的异步操作。而且，在for循环结束时，所有异步操作都将完成。

这用于await对for循环中的异步操作进行排序，因此必须将父函数标记为async，它将返回一个承诺，指示一切何时完成。